Supramolecular Hemoglobin-PEG Polymerization Equilibria as the Structural Basis for Providing Size-Controlled Polymer Particles.

Hemoglobin (Hb) can act as a dynamic supramolecular building block through the reversible dissociation of the α2β2 tetramer into αβ dimers. For this study, we designed structurally defined bicyclic monomers in which the β subunits in two Hb molecules are bridged through four-armed PEG of different molecular weights (10 and 40 kDa). These bicyclic monomers undergo concentration-dependent supramolecular ring-opening polymerization, enabling predictable control of polymer structure from discrete monomers to polymer particles and finally to a percolated polymer gel network. Extensive physicochemical analyses revealed quantitative shifts in polymerization equilibria that govern these supramolecular assemblies. Furthermore, intramolecular ββ-cross-linking of Hb moiety with bis-(3,5-dibromosalicyl) fumarate (DBBF) allowed selective covalent fixation of each supramolecular polymer species, affording fixed bicyclic monomers, fixed polymer particles, and fixed polymer gels with controlled sizes. These fixed Hb-PEG architectures retained characteristic O2-binding properties. Overall, this work establishes a supramolecular-to-covalent polymer conversion platform that enables precise structural control in Hb-based polymer architectures, thereby expanding the design space for protein-polymer biomaterials.